1. Technical Field
This invention relates to digital television (DTV) receivers and, more particularly, to a transmitter and receiver arrangement for a DTV receiver used to characterize significant, nearby indoor multipath reflectors.
2. Background Art
Digital television (DTV) is an emerging technology that is capable of providing video, audio and data services. The digital format allows for a great deal of flexibility in the content carried by DTV transmission. For example, High Definition Television (HDTV) service can be enabled using DTV. In the United States, research on HDTV has focused on digital, rather than analog, technology. While digital HDTV (or, more simply DTV) is not currently available, Japanese companies have developed an HDTV system based on analog technology (known as Hivision) that has been in use since 1991. Because of the potential advantages of digital HDTV and many technical problems shared by both types of systems, research in digital HDTV has also been active in Japan. See, for example, David K. Kahaner in xe2x80x9cHDTV Research in Japanxe2x80x9d, IEEE Micro, October 1993, pp. 49-53.
One of the most important prevalent problems in HDTV is the problem of multipath. In fact, it is useful to think of the HDTV channel as multipath limited and not power limited. Multipath may arise from fixed structures acting as reflectors in the transmission channel such as building walls. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, xe2x80x9cDesign of a Ghost Canceling Reference Signal for Television Systems in North Americaxe2x80x9d, Proceedings of Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, Canada, Sep. 14-17, 1993, pp. 660-663.
The effect of multipath is to create xe2x80x9cghostsxe2x80x9d in the displayed TV image. The statistics of multipath ghosts have been studied and compiled by, among others, the BTA (Japan""s Broadcasting Technology Association). A BTA survey reported that 92% of ghosts are within a xe2x88x924 to 26 microsecond (xcexcsec) range, and when extended to xe2x88x924 to 37 xcexcsec, almost all cases are covered.
An adaptive equalizer has been proposed to xe2x80x9cundoxe2x80x9d the effects of the multipath. In its crudest form, an adaptive equalizer can be thought of as a signal processor that estimates the parameters of a hypothetical filter that best describes the channel. The signal processor adjusts the taps of the adaptive equalization filter to approximate an inverse of the hypothetical filter, thus inverting, or undoing, the effects of the multipath.
The BTA, and other concerns, designed a xe2x80x9cghost canceling reference (GCR)xe2x80x9d transmitted signal to mitigate these multipath induced effects. The BTA GCR was found to be less than satisfactory in some cases. While homes with outdoor antennas displayed non-varying (stationary) ghosting conditions which could be largely corrected, those homes with indoor antennas experienced changing (dynamic) ghosts. These ghosting conditions were more prevalent where people were moving about the room or moving bodies were in the signal path. The BTA ghost canceller generally was not able to adequately compensate for these conditions. In fact, false ghosts were actually added to an already ghosted picture leading to reduced picture quality.
Thus, multipath behavior of the HDTV channel is important for two different regimes, the outdoor antenna propagation channel and the indoor antenna propagation channel. The former is well studied and understood. The latter situation still presents a problem. The chief difference is the presence of significant reflectors near the indoor receiving antenna, which implies that there will be multipath whose delay occasions it to fall within a symbol period. In order to resolve multipath differences of such limited extent, special techniques must be employed or the channel diagnostic signal must have a very wide effective bandwidth. According to S. Salous in xe2x80x9cIndoor and Outdoor UHF Measurements with a 90 MHZ Bandwidthxe2x80x9d, IEEE Colloquium on Propagation Characteristics and Related System Techniques for Beyond Line-of-Sight Radio, 1997, pp. 8/1-8/6, the extent of multipath delays in outdoor environments can be a few tens of xcexcsecs, whereas in indoor environments, multipath delays are on the order of a few hundred nanoseconds. While multipath components can be adequately resolved with a 10 to 40 MHZ bandwidth for outdoor environments, the resolution of multipath for indoor environments requires 90 to 100 MHZ bandwidth.
A significant, nearby indoor multipath reflector is characterized by providing a DTV receiver with a sounder comprised of a special transmitter and a plurality of separately located receivers, each receiver including a respective transducer or antenna, so that the sounder, in effect, comprises a bistatic radar system for any given channel. A microprocessor receives the timing information generated by the transmitter impulses and the reflected returns to each of the receiving transducers or antennas, and calculates a multipath model representing the three-dimensional location of a nearby scatterer.
The location of the nearby scatterer enables, as an ancillary feature, adaptive nulling of a reflected or scattered (multipath) signal from a significant nearby scatterer. Depending upon the characteristics of the scatterer, this multipath signal may create flat (frequency) fading of the DTV signal received by the antenna of the DTV receiver. If the DTV receiver is equipped with a multi-element receiver aperture, wherein each element is individually controllable as to gain, knowledge of the relative position of the significant scatterer to the DTV receiver antenna allows a null in the antenna aperture to be steered in the direction of the nearby significant scatterer, thus mitigating the multipath signal and mitigating the flat fading condition. The result is a higher signal-to-noise ratio at the front-end of the DTV receiver with a concomitant increase in picture quality.